1) Field of the Invention
The present invention relates to an apparatus for measuring stress of viscoelastic material such as rubber, by which the stress generated in the viscoelastic material when a shear is continuously given thereto can be measured.
2) Prior Art Statement
The performance of rubber products is determined and influenced by the characteristics of raw material and rubber compounded thereinto, such as plasticity, viscosity and elasticity. Therefore, when producing, evaluating, developing and researching an elastomer element or a composition of rubber, it is necessary to measure such characteristics of the materials and obtain exact information concerning processing efficiency thereof.
M. Mooney invented a rotary disc type viscometer 60 years ago which is used for measuring a viscocity of viscoelastic material. This viscometer is widely used for obtaining indices of processing efficiency of the elastomer such as rubber; in the rubber industry. The viscometer is standardized by ASTM and ISO, etc. throughout the world. In Japan, it is standardized in JIS K6300 such that the physical property of non-vulcanized rubber is measured by using the Mooney viscometer.
FIG. 1A is a cross-sectional view showing the Mooney viscometer. The Mooney viscometer is a so-called rotor type viscometer, in which a rotor 101 and sample material are arranged in a cylindrical sample chamber 103 which is formed by an upper die 102a and a lower die 102b. The rotor 101 is rotated in one direction by a motor (not shown) to measure a counter-torque which acts on a shaft 101a of the rotor 101. The Mooney viscometer is widely and generally used and has improved over time. However, even in the improved viscometers, the rotor is always used. Therefore, the Mooney viscometer has such problems that: 1 the measurement data is varied due to a bending of the rotor shaft 101a; 2 generally two or more rotors are used in one Mooney viscometer apparatus in order to increase operational efficiency and thus the measurement data is varied between or among the plurality of rotors; 3 when the Mooney scorching test is conducted, particularly, the side surfaces and the upper surface of the rotor 101 are damaged when the sample is removed from the rotor after the test has been finished, so that the measurement data is varied; and 4 the measurement data is varied due to an aging of an O ring (not shown), which is arranged on the rotor shaft 101a for the purpose of sealing and due to a friction force generated between the O ring and the rotor shaft 101a. Therefore, in order to keep measurement accuracy of the Mooney viscometer, it is necessary to increase the number of steps for maintenance of the viscometer.
By the Mooney viscometer, a rotation number of the rotor 101 and a value of counter torque, which acts on the rotor shaft 101a, are measured. The method is widely used in that the rotation number and the value of counter-torque are exchanged into a shearing velocity (.gamma.) and a shearing stress (s) to evaluate the processing efficiency of the sample. In this case, it is necessary to certainly give a shearing force to the sample without slipping of the rotor 101 and to correctly detect the shearing stress which acts on the sample as the counter torque. FIG. 1B is a graph showing, a shearing velocity (.gamma.) given to the sample, which is measured in the Mooney viscometer shown in FIG. 1. In the graph of FIG. 1B, the abscissa R represents a distance from a center point of the rotor 101. The sample contained in the sample chamber 103 is divided into characteristic regions of a, b and c, as shown in FIG. 1C, to measure separately the shearing velocity and the shearing stress in each region. On inner surfaces of the upper and lower dies 102a and 102b and all of outer surface of the rotor, a plurality of grooves are formed in order to prevent slipping of the sample, so that the rotation of the rotor certainly gives a shear to the sample and the shearing stress generated in the sample thereby is transmitted to the rotor shaft 101a as the counter torque. However, in the apparatus shown in FIG. 1C, it is impossible to detect the shearing stress of the sample arranged in the region c. Because, there has been no formula to exchange the rotation number of the rotor shaft 101 into the shearing velocity concerning the sample arranged in the region c; and since the sample arranged in the region c is not in contact with the rotor 101 directly it can be assumed that the shear which acts on the sample c does not contribute to the torque directly.
There is disclosed a conical type rotating viscometer, i.e. a rotorless type viscometer, in Japanese Patent Publication No. 50-26101. FIG. 2A is a cross-sectional view showing a principle construction of the rotorless type viscometer. The rotorless type viscometer comprises an upper die 104 on the surface of which a plurality of grooves are formed in order to prevent the slip of the sample, a lower rotational die 105, a seal 104a made of elastic material arranged between the upper die 104 and an upper fixed die 109, an annular ring 106 arranged around the outer circumference of the lower rotational die 105, and a lower die protection ring 107 arranged around the annular ring 106. It should be noted that these members are arranged in a concentrical manner. The annular ring 106 and the lower die protection ring 107 are arranged to be movable in upper and lower directions by means of springs 106a and 107a whose end portions are secured to a flange portion 105a of the lower rotational die 105.
A heater 108 is embedded in the lower rotational die 105 and to the heater 108 electric power is supplied via slip rings 108a. A top portion of the lower die protection ring 107 is engaged with an angular groove 109a which is formed on the bottom surface of the upper fixed die 109. As shown in FIG. 2A, the sample chamber 110 is formed by the inner surface of the lower die protection ring 107, the bottom surface of the upper die 104, the bottom surface of the upper fixed die 109, the upper surface of the annular ring 106, and the upper surface of the lower rotational die 105. The shear is given to the sample contained in the sample chamber 110 by rotating the lower rotational die 105 and the lower die protection ring 107; the torque generated in the sample is measured by the upper die 104.
In order to make an elasticity transformation torque as small as possible, a portion of the surface of the seal 104 arranged around the outer circumference of the upper die, i.e. die for detecting the torque, is exposed to the sample chamber 110. However, the surface of the seal 104 exposed to the sample chamber 110 is not formed such that the slip of the sample can be prevented, while the protection ring 106 is arranged to be rotatable being accompanied with the rotation of the lower rotational die 105. Therefore, when the lower rotational die 105 is rotated, the sample arranged under the outer circumferential portion of the upper die 104 and the upper fixed die 109 suddenly slips, so that the shearing velocity in the vicinity of the outer circumference of the upper die 104 falls into disorder. FIG. 2B is a graph showing the velocity of the shear generated in the sample contained in the sample chamber 110 of the apparatus shown in FIG. 2A. It is clear from FIG. 2B that the counter torque in accordance with the shearing stress being influenced by the slip of the sample arranged under the exposed portion of the seal 104a exposed to the sample chamber 110 is measured by the upper die 104. The slipped sample is not given a shear to the upper die 104 but rotated at the same rotating speed as that of the lower rotational die 105.
As a result, the shearing velocity of the sample arranged in the vicinity of the outer circumference portion of the upper die 104 is high and an apparent large counter-torque is measured by the upper die 104. Further, the slippage amount of the sample cannot be specified because it is also varied in accordance with a tackiness between the sample and the inner wall of the sample chamber 110.
According to linear elastic theory, the counter torque T generated in the torque detecting die is represented by the formula T.varies.R.sup.3 in a conical type die or the formula T.varies.R.sup.4 in a parallel disc type die; wherein R represents a radius of the torque detecting die. As is clear from this, since the outer circumferential portion of the torque detecting die largely contributes to the counter-torque generated therein, the disorder of the shearing velocity in the outer circumferential portion of the torque detecting die would influence the torque to be detected.
Furthermore, in this apparatus, the torque detecting die is rotated by the shearing stress given by the sample; the force to rotate the torque detecting die is detected as the torque. However, the value of the torque detected by the torque detecting die does not include a friction torque generated between the outer circumferential surface of the torque detecting die 104 and the inner surface of the seal 104a and an elasticity transformation torque of the seal 104a. Therefore, the torque detected by the torque detecting die does not show the correct value.
Furthermore, the sealing effect of the seal 104a deteriorates due to the heat deterioration of the material thereof when long time has been passed. Additionally, the friction torque and the elasticity transformation torque are apt to be increased, accordingly. Moreover, since the increased values of the friction torque and the elasticity transformation torque cannot be specified, it is difficult to correct the deteriorated value.
In the Japanese Patent Publication No. 60-25735, there is disclosed another rotorless type hardness measuring apparatus. FIG. 3A is a cross-sectional view showing the rotorless type hardness measuring apparatus. The apparatus comprises an upper die 111, a lower die 112, an upper fixed die 113 and a lower fixed die 114. A sample chamber 115 is formed by the lower surface of the upper die 111, the upper surface of the lower die 112, the lower surface of the upper fixed die 113 and the inner surface of the lower fixed die 114. It should be noted that there are formed a plurality of grooves on the lower surface of the upper die 111 and the upper surface of the lower die 112. A sample is contained in the sample chamber 115; the lower die 112 is rotated about a rotating shaft of the lower die 112. Therefore, the shear having an amplitude of small angle is repeatedly given to the sample, and thus the disorder due to the slip of the sample does not generated so much. However, when the lower die 112 is rotated in one direction, i.e. the shear in one direction is given to the sample continuously, the shearing value is increased as time goes on and the sample arranged in the vicinity of an O ring 116, which is arranged between the upper die 111 and the upper fixed die 113, is slipped to the outer side, so that the shearing velocity in the vicinity of the seal portion 116 falls, as shown in FIG. 3B.
In the above mentioned measurement apparatuses, there are provided heating devices in the upper and lower dies in order to keep the sample chamber at an appropriate temperature. One example of the heating device is shown in FIG. 2A. In the apparatus shown in FIG. 2A, heating devices 108 and 110 are embedded in the upper and lower die 104 and 105, respectively, and the temperature in the sample chamber 110 is controlled by these heaters. Since the heating device 108 provided in the rotational die 105 is rotated in accordance with the rotation of the lower die 105, slip rings 108a are generally used to supply electric power to the heating device 108. Additionally, a temperature detector (not shown) is generally arranged inside of the lower die 105, and the temperature signal thereof is generally derived via slip rings 108a.
However, in the conventional apparatus there are many problems such that electric contacts at the slip ring portions sometimes deteriorate, the contact register is varied and electric noise is generated, due to corrosion of the slip ring portion, dust adhered on the slip ring portion or abrasion of the carbon brush. And thus the deteriorated electric contact, the decrease of the contact register and the electric noise sometimes disable the apparatus and decrease the accuracy of temperature controlling.
Further, in the conventional apparatus, the heating devices are directly embedded in the upper and lower dies, so that the upper and lower dies are directly heated up by the heating devices. Therefore, the temperature of these dies are not even; that is to say, the temperature of the portion close to the heating device becomes high and that of the portion far from the heating device becomes low. The unevenness of the temperature of the dies results that the measurement data of the viscocity of the sample is varied because the temperature of the sample heated up by the dies becomes heated up in an uneven manner.
In Japanese Patent Publication No. 60-120253, there is disclosed an apparatus in which the sample is automatically mounted in the sample chamber and automatically removed therefrom. However, if the automatic mounting and removing system is applied to detect the hardness of a non-vulcanized rubber or a semi-vulcanized rubber, it is difficult to remove the sample, i.e. non-vulcanized or semi-vulcanized rubber, from the sample chamber after the measurement of the hardness of the sample has been finished, because the non-vulcanized or semi-vulcanized rubber has a high viscosity.